Any UFO investigator who presumes to evaluate electronic evidence should have much more than an amateur’s knowledge of the nature and behavior of radar. Correct interpretation of the signals requires training, experience, skill, and an expert’s acquaintance with the peculiarities of the set under varying conditions. But even the expert does not yet understand the causes of all the phenomena that can appear. He is limited by our still incomplete knowledge of dynamic meteorology--precise information about the composition of the atmosphere and how it interacts with microwaves. With proper instrumentation and first-rate operators, radar can correctly report the approximate direction, distance, altitude, and rate of motion of objects within its range. If the returns are misinterpreted, however, radar can seem to give false reports. [Illustration: _Figure 13._ Schematic view of radar targets on successive sweeps of the antenna.] Radar is not a TV camera or a photographic lens. It does not, at least at present, produce a picture of the physical appearance, shape, size, or color of the thing it detects. The scope shows only tiny spots of light on the flat surface of a screen. A pointer something like a clock hand continually sweeps around the dial at a given speed. A complete rotation may take from two to fifteen seconds, depending on the type of the set. This sweep hand keeps pace with the rotation of the radar antenna as it scans the sky by sending out radio pulses. When they encounter a solid object, they bounce off and return to the set as echoes which show as “blips,” or spots of light, on the radarscope. The operator must interpret these spots and try to identify them as planes, helicopters, balloons, ships, mountains, clouds, birds, storms, hurricanes, or phantom echoes of various kinds. Safe commercial flying depends on the accuracy of these identifications, as does the security of the country in periods of international tension. Radar only reports. It does not interpret. If the sweep hand on successive rotations shows a spot of light apparently moving from position _A_ to position _B_, to _C_, to _D_, the operator generally concludes that the blips represent a single object that is moving at a certain speed in a certain direction (see Figure 13). If successive sweeps show a spot of light that remains at position _A_, he usually concludes that it represents a stationary object. If the blip moves a very great distance in the interval between two sweeps or seems to jump erratically from one position to another, an amateur might interpret it as a spacecraft flying at incredible velocity--a flying saucer. But an expert would probably conclude, especially under certain weather conditions, that the scope was picking up echoes from two or more separate objects, one reflecting briefly at position _A_, another at position _B_, and so on. _The Principle of Radar_